Use a trivial solution to #1605

Signed-off-by: György Krajcsovits <gyorgy.krajcsovits@grafana.com>

prometheus/histogram.go

@@ -1682,13 +1682,13 @@ func makeNativeExemplars(ttl time.Duration, maxCount int) nativeExemplars {
 }
 
 func (n *nativeExemplars) addExemplar(e *dto.Exemplar) {
+	n.Lock()
+	defer n.Unlock()
+
 	if cap(n.exemplars) == 0 {
 		return
 	}
 
-	n.Lock()
-	defer n.Unlock()
-
 	// The index where to insert the new exemplar.
 	var nIdx int = -1